Fluorescent silicon nanoparticles have been produced in a two-step process in ultra high vacuum. First, silicon clusters were produced in the gas phase in a molecular beam. At the end of the cluster beam machine the cluster were co-deposited with water onto a cold target. Melting of the ice yields a suspension that fluoresces at 420 nm when excited with ultraviolet light. The fluorescence intensity remains constant over a period of more than a year. Photo-absorption and photo-luminescence spectra provide evidence of a Si/SiO2 core-shell structure having a silicon core size of at least 1.4 nm in diameter and oxygen deficient O-Si-O defects as the origin of the deep-blue fluorescence. Furthermore, the fluorescent suspension was deposited on freshly cleaved highly oriented pyrolytic graphite (HOPG). AFM images recorded in UHV showed networks of agglomerated clusters, their smallest units having a diameter of typically 0.7 nm.